Asteroid protoplanet may hold clues to Earth formation

Never have old leftovers made so many scientists salivate. After a four year journey, NASA's Dawn spacecraft has returned the first close-up views of the giant asteroid Vesta, a relic of planet-building that could hold clues to how Earth formed.

At 530 kilometres across, Vesta is one of the biggest denizens of the asteroid belt, the junkyard of leftover planetary building blocks found between Mars and Jupiter.

For most of Dawn's journey, Vesta appeared as no more than a star-like speck in the sky. But Dawn is now close enough to return the best views of Vesta yet, surpassing the detail available in Hubble Space Telescope portraits.

Last week, NASA showed off the images to the media and described what Dawn will do to investigate Vesta while in orbit.

Early growth

"Vesta is a window into the early origins of our solar system and the terrestrial planets," said Dawn team member Carol Raymond of NASA's Jet Propulsion Laboratory in Pasadena, California. "We're on the edge of our seats waiting for this data to come in."

The pictures snapped by Dawn since mid-May are still blurry, showing only tantalising hints of craters on Vesta's surface.

But as the craft slips into orbit around Vesta in mid-July, cameras and spectrometers will reveal its topography and chemical composition. It's thought that Vesta finished growing long before Earth and the other planets, so might preserve clues to what that era of early planet formation was like.

After a year in orbit, Dawn will head to Ceres, the solar system's biggest asteroid. Unlike Vesta, Ceres seems to contain a lot of water-ice, and scientists hope data collected by Dawn will help them understand how the two large asteroids ended up so different in composition.

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A window ino the early solar system (Image: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)